Polyphenylene ether) resin is a type of plastic known for its excellent water resistance, dimensional stability, and inherent flame retardancy, as well as high oxygen permeability and oxygen/nitrogen selectivity. Properties such as strength, stiffness, chemical resistance, and heat resistance can be tailored by blending it with various other plastics in order to meet the requirements of a wide variety of consumer products, for example, plumbing fixtures, electrical boxes, automotive parts, and insulation for wire and cable.
The most commercially important polyphenylene ether is currently poly(2,6-dimethyl-1,4-phenylene ether), which is prepared on a large scale by the oxidative polymerization of 2,6-dimethylphenol (also known as 2,6-xylenol). For certain product applications, notably use in hollow fiber membranes, very high molecular weight poly(2,6-dimethyl-1,4-phenylene ether)s are needed. Not only must the average molecular weight be very high, but the sample must have a small weight percent of low molecular weight polymer chains. There is therefore a need for poly(2,6-dimethyl-1,4-phenylene ether)s that have but a high number average molecular weight and a reduced fraction of low molecular weight molecules.
The literature includes various procedures for the preparation of high molecular weight poly(2,6-dimethyl-1,4-phenylene ether)s, but these procedures are deficient for one reason or another. For example, U.S. Pat. Nos. 4,110,311 and 4,116,939 to Cooper et al. require the addition of chemical compounds that interfere with the solvent recycling that is required to make commercial process environmentally acceptable. Specifically, U.S. Pat. No. 4,110,311 requires the addition of a dihydric phenol and a mild reducing during the copper removal step, and U.S. Pat. No. 4,116,939 requires the addition of an aromatic amine. As another example, U.S. Pat. No. 6,472,499 B1 to Braat et al. provides a commercially viable process for producing high molecular weight poly(2,6-dimethyl-1,4-phenylene ether)s, but the present inventors have observed that the products of the Braat et al. process, once isolated, have a relatively large fraction of lower molecular weight molecules. Other references provide procedures that are suitable for use on a laboratory scale, but for reasons that are not always well understood it is difficult or impossible to successfully translate the procedures to a commercial scale. It should also be noted that many references characterize product poly(2,6-dimethyl-1,4-phenylene ether)s in terms of intrinsic viscosity, rather than molecular weight, and there is no way to deduce a specific molecular weight distribution from an intrinsic viscosity value.
There therefore remains a need for poly(2,6-dimethyl-1,4-phenylene ether)s having high number average molecular weight and reduced content of molecules with low molecular weight (e.g., polymer chains with a molecular weight less than 30,000 atomic mass units). There is also a need for improved, commercially scalable, and environmentally acceptable processes for producing such poly(2,6-dimethyl-1,4-phenylene ether)s.